High frequency structure



8, 1959 w. G. SHEPHERD, 2,900,560

HIGH FREQUENCY STRUCTURE Filed Aug. 12, 1954 2 Sheets-Sheet 1 F|G.5 W F|G.8

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Aug. 18, 1959 we. SHEPHERD 2,900,560

HIGH FREQUENCY STRUCTURE Filed Aug. 12. 1954 2 Sheets-Sheet 2 INVENTOR. W/LL/AM G. SHEPHERD A TTORNE Y United States Patent O HIGH FREQUENCY STRUCTURE William Gerald Shepherd, St. Paul, Minn., assignor to Bendix Aviation Corporation, Eatontown, N.J., a corporation of Delaware Application August 12, 1954, Serial No. 449,337

2 Claims. (Cl. 315-5.19)

The present invention relates to electron discharge devices and more particularly to devices adapted for use at ultra high frequencies.

In electron discharge devices which make use of cavity resonators, as the frequency is increased the geometry of the device decreases. This tends to limit the electron discharge currents that can be handled by the device and limits the power output thereof. Further, the small size as dictated, by the frequency makes construction difficult and dimensions extremely critical.

The present invention provides an electron discharge device that is capable of handling high discharge currents at ultra high frequencies and permits expanding the physical size thereof. The device uses a waveguide which is loaded at its center by a ridge. For a given width of the guide, the center loading has the effect of reducing the frequency at which the guide is cut ofi. Optimum performance is obtained by operating the ridge loaded section at cutoff with the ends of the guide beyond the ridge beyond the cutoif.

It is an object of the invention to provide an improved resonant cavity.

It is another object of the invention to provide an improved high frequency structure. I

Another object of the invention is to provide a novel high frequency electron discharge device capable of handling high discharge currents.

Another object of the invention is to provide a novel resonant cavity adapted for operation .at the higher modes.

The above and other objects and features of the invention will appear more fully hereinafter from a consideration of the following description taken -in connection with the accompanying drawing wherein several embodiments of the invention .are illustrated by way of examples.

In the drawing:

Figure 1 is a partial cutaway view of a reflex cavity embodying the invention.

Figure 2 is a cross section view of the device illustrated in Figure 1.

Figure 3 is a cross section view of two cavities connected as an amplifier.

Figure 4 is a cross section view of two cavities adapted for a grid control.

Figure 5 is a diagram illustrating the electric field distribution of the TB mode.

Figure 6 is a diagram illustrating the electric field distribution of the TEgo mode.

Figure 7 is a graphic plot of the electric fields of the modes of Figures 5 and 6.

Figure 8 is a series of curves illustrating graphically the electric fields of several modes.

Figures 9 to 13 illustrate diagrammatically various methods of tuning the device.

Referring now to the drawing, wherein the same reference numerals have been assigned to similar parts in the various figures, a reflex oscillator is indicated generally by the numeral 10. It is understood that the oscilr 2,900,560 Patented Aug. 18, 1959 7 an upper face 16. Integral with the lower face 15 is a hollow ridge 17 terminating in a grid section 18. The cavity 11 is so proportioned that for a frequency at which the ridge portion is at cutofi, the end sections will be beyond cutoff.

Opposite the grid 18 is a grid 20 in the upper face 16. The repeller 13 is mounted on the upper face 16 opposite the grid 20 by insulating members 21.

The electron gun 12 may be of the type having a cathode 22, focussing electrode 23 and beam electrode and heat shield 24. The cathode 22 has an electron emitting surface 25 extending substantially throughout the length of the ridge 19. A heater or filament 26 may be provided for the cathode 22. The focussing electrode 23 is located between the cathode 22 and the cavity 10 and coacts with the beam electrode 24, which partially envelopes the cathode 22', so as to form a converging line focus beam which will cross the gap between the grids 18 and 20. The cathode 22, focussing electrode 23 and beam electrode 24 may be supported and positioned relative to the cavity by suitable insulator supports such as mica or ceramics (not shown).

The output may be taken by means of a coaxial line 27 terminating in a coupling loop 28 in the cavity 10. It is understood, however, that other means may be employed for coupling the output of the cavity.

In operation, such for example as a reflex oscillator, a disturbance of a transient nature in the cavity 10 at the cutoff frequency will velocity modulate the electron stream between the grids 18 and 20. This velocity modu-. lation will be converted to a convection current modulation between the grid 20 and the repeller electrode 13. By proper adjustment of potential, the beam returns through the gap at proper phase and develops an oscillation at the cutoff frequency of the cavity. Inasmuch as the end sections are beyond cutoff, the power loss from leakage will be negligible.

Normally for frequencies higher than the cutoff frequency for the center section, the end sections will not be cutoli. For these higher frequencies a leakage of RF. power out of the ends will occur causing heavy loading, tending to inhibit oscillation at these frequencies. Further, for those higher frequency modes, a resonance condition of the guide will require that a variation of the RF. field occur along the axis of the guide. In other words it will vary sinusoidally along the length so that the field with which an electron stream interacts will not be uniform over most of the length resulting in a less eflicient interaction than for the desired mode. Also, the modulation coefiicient for these higher frequency modes will be smaller than for the cutoff frequency thereby further reducing the efliciency of interaction between the electron stream and the field.

In the case of modes which vary longitudinally, cuts in the cavitytransverse to the axis may be utilized to cause circuit loss. These cuts will not affect the cutofi mode as the currents in this mode will not traverse the cuts.

It may be desirable to operate the cavity in certain higher modes at cutolf. This can be accomplished by of a higher mode, for example the T13 mode, in the central or ridge section of a cavity having its lateral dimensions expanded for operation in the TE mode Wlth the end sectionsbeyond cutoff for this mode but for higher modes would not be beyond cutoff.

In operation in the higher modes there will be at least one lower frequency mode for which the end sections will be beyond cutoff. For example, in operation inthe TE mode, there will be a lower frequency TE mode for which the end sections will be beyond cutoff. Referring now to Figure 7, the curve A is a graphic representation of the electric field of the T13 mode, and the curve B is a graphic representation of the electric field of the TE mode as they appear in the expanded cavity of Figure 6. Oscillations at the TE mode can be prevented by providing lossy strips axially along the guide in planes indicated by the dashed lines XX: For the TE mode, the lossy strips will be at minima of the electric field, hence will have a minimum effect on the operation at that mode. It is also possible to discriminate against the TE mode by adjusting the drift distance in the repeller space so that the electronic admittance developed in shunt with the gap is unfavorablefor the low frequency TE mode while it is favorable for the TE mode. Thus the TB mode can be inhibited by the choice of operating parameters.

For a better understanding of the invention reference is made to Figure 8 which illustrates graphically the electrical fields of the various modes as seen in a waveguide which is assumed to be beyond cutoff at its ends for all modes. The damped fields in the cutoff ends are not shown. Figure 8A illustrates graphically the uniform electric field pattern for the TB mode at cutoff.

The electric field for the TE mode, as illustrated in Figure 88, would be objectionable as it is not uniform throughout the length of the line. Cuts normal to the guide axis will inhibit the TE mode.

The TE mode, as illustrated in Figure 8C, will not be excited since its electric field is zero at the center.

The TE Figure 8D, like the TE mode would be objectionable but since it is not uniform throughout the length of the line, it will interact less strongly with the electron discharge. It also can be inhibited by cuts normal to the axis of the guide.

The T13 as a desired mode, is illustrated in Figure 813. For this mode, the lossy strips used for inhibiting the TE mode will have little effect as they will be at the minima of the electric field.

Figure 3 illustrates a klystron amplifier in which cavities 11 and 11A are positioned with the grids 20 and 20A in alignment. A drift region 31 separates the cavities. The cavity 11A is provided with a collector 32. An input signal is fed into the cavity 11 by means of the line 27 and the output is taken from the cavity 11A by means of the line 27A.

Figure 4 illustrates the use of two cavities for grid control. In this arrangement, the cavities 11 and 11A are positioned adjacent each other with the grids 20 and 20A in alignment. A cathode '22 is provided for the cavity 11 and is insulated and bypassed therefrom.

For some applications it is desirable to be able to tune the cavity over a band of frequencies. For example the cavity may be tuned by varying the gap spacing, by inserting a dielectric vane to capacitively load the gap or in the case of the TE mode to load at other maxima of the electric fields by inductive loading of the cavity.

Referring now to Figure 9 wherein flexible diaphragm members 33 are inserted in the top wall 16 of the cavity 11. The grid 20 is supported between the diaphragm members 33 by sections 34 of the wall 16. The repeller 13 is supported by a long inverted U-shaped insulating member 35. The legs 36 of the member 35 are secured to the sections 34 by cementing or any other conventional manner. A threaded member 37 has one end secured in the member 35 with the other end extending through opening 38 in bracket 39. The bracket 39 may be secured to the cavity 11 by screws or in any other suitable manner. An adjusting nut 40 secured in the opening 38 is provided for engaging the member 37. Rotation of the nut 40 causes adjustment of the gap between the grids 18 and 20 and thereby the resonant frequency of the cavity.

Figure 10 illustrates means for tuning the cavity by capacitively loading the gap. Dielectric vanes 41 are hinged to the wall 16 by a suitable means. Suitable mechanical means may be provided for linking the vanes together outside of the cavity.

In the means illustrated in Figure 11 the vanes 41 are secured to members 42 which may be actuated from the outside of the cavity to move the vanes along the longitudinal axis of the cavity. As the vanes 42 approach the gap, the resonant frequency is lowered.

Figure 12 illustrates a further method of tuning the cavity 11 in which a block of dielectric material 43 is movable by a member 44 in the cavity. While only a portion of the cavity is illustrated it is understood that the tuning means would be symmetrical and that a similar block would be on the other side of the gap.

Figure 13 illustrates means for tuning the cavity by inductive loading. A slug of magnetic material 45 is mounted on a plunger 46 and adapted for movement in the magnetic field of the cavity. Likewise, it is understood that the tuning would be symmetrical and slugs would be provided for both sides of the gap.

Although only a few embodiments of the invention have been illustrated and described, various changes in the form and relative arrangement of the parts, which will now appear to those skilled in the art, may be made without departing from the scope of the invention.

What is claimed is:

1. A reflex klystron comprising a rectangular cavity having an internal inverted V-shaped ridge section in one Wall thereof, said ridge section terminating in a grid, a second grid in the wall of said cavity opposite said ridge, said grids being parallel and in alignment with each other to form a gap therebetween, said cavity being proportioned to be at cutoff in said ridge section for a predetermined frequency with end sections beyond said ridge section being proportioned to be beyond cutoff for said frequency and having its lateral dimensionsexpanded for operation at a higher mode, an electron gun including an elongated cathode positioned exteriorly of said cavity adjacent to and in alignment with said ridge section, and a repeller electrode positioned exteriorly of said cavity adjacent to and in alignment with said other grid.

2. The combination as set forth in claim 1 and including means for inhibiting the unwanted modes of said predetermined frequency.

References Cited in the file of this patent UNITED STATES PATENTS Lerbs Sept. 24, 1957 

